Welp, let's let redirection work too

[DarthHater]

In #92 we discovered that Nancy would only accept piped stdin, not redirected. This PR aims to fix that!

This pull request makes the following changes:

• Adds a new checkStdIn() function to remove some duplicate code
• Maintains same behavior of previous StdIn check, but uses os.ModeCharDevice to check if something is in StdIn

It relates to the following issue #s:

• Fixes #92

cc @bhamail / @DarthHater / @zendern / @fitzoh / @AndreyMZ